There has recently been an increasing demand for galvannealed steel sheets as the rust-proof steel sheets for automobiles, since they exhibit high corrosion resistance and weldability when painted. The latest tendency has been toward sheets having a heavier c/w to ensure high corrosion resistance.
These galvanized steel sheets are required to have excellent press-formability and exhibit excellent anti-powdering property when press formed. These requirements have lately been becoming more stringent, and the increasing coating weight has been creating a big problem in the maintenance of, above all, excellent anti-powdering property.
There is known a process which comprises heating galvanized steel sheets rapidly to cause the alloying of a part of coating, and batch annealing them to improve their anti-powdering property. This process is effective in achieving an improved anti-powdering property, but has the drawback of being expensive.
Japanese Laid-Open Patent Application No. Hei 1-279738 discloses a process for achieving an improved anti-powdering property in line. According to its disclosure, steel sheets are plated in a bath containing 0.04 to 0.12% A1, are heated to a temperature of at least 470.degree. C. rapidly within two seconds to undergo alloying, and are cooled to a temperature not exceeding 420.degree. C. rapidly within two seconds, whereby galvannealed steel sheets consisting mainly of a .delta..sub.1 phase are manufactured.
Due to the relatively high temperature which the process employs for the alloying treatment, however, it is very likely that alloying may proceed so rapidly that the growth of a thick .GAMMA. phase may result in a low anti-powdering property. Although Japanese Laid-Open Patent Application No. Hei 1-279738 proposes rapid cooling within two seconds from the temperature range in which alloying is effected, to the temperature range not exceeding 420.degree. C. to prevent excessive alloying, a proper alloying pattern varies with the coating weight and the line speed, and the use of the process, therefore, calls for the provision of a multiplicity of sources of heating and cooling mediums along a line and thereby brings about an increase in the cost of equipment.
Moreover, a direct gas-fired alloying furnace which is usually employed is likely to have a temperature variation along the width and length of a steel strip, and such temperature variation makes difficult the strict control of the coating structure as hereinabove stated and results in the formation of a coating having excessively alloyed portions or containing a residual .zeta. phase. The resulting plated steel sheet lacks uniformity in the amount of its .delta..sub.1 phase and therefore in its anti-powdering property. The amount of the .zeta. phase has so close a bearing on the frictional properties that those portions of the plated steel sheet which contain the residual .zeta. phase have a higher frictional coefficient and are, therefore, lower in press formability.